The invention relates generally to comparators, and more particularly to a differential window comparator.
A zero-crossing comparator is a common circuit element typically formed with a high-gain differential amplifier that receives two independent input voltages and produces an output voltage signal proportional to the voltage difference between the two inputs, amplified according to the high gain. Such comparators are known as fully differential and will go into positive or negative saturation depending upon the relative difference of the input voltages. In this fashion, the output voltage signal indicates which one of the input voltages is greater than the other.
A window comparator is a special type of comparator that may produce a saturated output if the absolute value of the difference between the two input signals exceeds a specified value. There are two threshold voltages for such a window comparator: one at the upper edge of the window and the other at the lower or most negative voltage. Typically, a window comparator may comprise two zero-crossing comparators, where one is for the positive threshold and the other is for the negative threshold, combined with an OR function. Such window comparators require a relatively large number of transistors. Alternatively, window comparators may use a single comparator coupled to xe2x80x9csingle-endedxe2x80x9d differential stages to give a more compact design. However, the window comparator will then suffer from higher common mode noise sensitivity.
Accordingly, there is a need in the art for a compact design for a fully differential window comparator.
In accordance with one embodiment of the invention, a window comparator is fully differential with respect to a first and a second voltage input. The window comparator includes a first differential pair, wherein a first transistor in the first differential pair is biased to conduct a first current and a second transistor in the first differential pair is biased to conduct the first current when the difference between the first input voltage and second input voltage equals a first voltage threshold. In addition, the window comparator includes a second differential pair, wherein a first transistor in the second differential pair is biased to conduct a second current and a second transistor in the second differential pair is biased to conduct the second current when the difference between the first input voltage and second input voltage equals a second voltage threshold. A zero-crossing comparator has a first input coupled to a first terminal on the first transistors in the first and second differential pairs and a second input coupled to a first terminal on the second transistors in the first and second fully differential pairs.
A more complete understanding of embodiments of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more embodiments. Reference will be made to the appended sheets of drawings that will first be described briefly.